Abstract: Melatonin (N-acetyl-5-methoxytryptamine) is known as the hormone of darkness because it is synthesized at night and involved in regulating the circadian clock. The hormone is primarily synthesized by the vertebrate pineal gland, but is ubiquitous among invertebrates, unicellular organisms, plants, and even cyanobacteria (Hattori and Suzuki, 2024). Melatonin is well-conserved evolutionarily and possesses several physiological functions, such as immune response, bone and glucose metabolism, and memory formation besides regulating the circadian rhythm. In mammals, G-protein–linked melatonin membrane receptors are present in the brain, retina, spleen, spinal cord, intestine, kidney, prostate, ovary, skin, muscle, and liver. Nuclear receptors are also present in many mammalian tissues, implying the widespread physiological actions of melatonin via receptors. Moreover, melatonin has established functions as an endogenous free radical scavenger and as a broad-spectrum antioxidant (Galano and Reiter, 2018). Altogether, these findings demonstrate that melatonin has diverse physiological functions, which are thought to be supported by evolutionary relationships. Furthermore, melatonin metabolites have been observed to have several functions similar to those of melatonin. Melatonin metabolites, including 3-hydroxymelatonin, 2-hydroxymelatonin, N1- acetyl-N2-formyl-5-methoxykynuramine (AFMK), or N1-acetyl-5-methoxykynuramine (AMK), have been detected among species. In this perspective, we introduce the function of melatonin metabolites, notably focusing on the memory formation–related function of AMK.